/src/openssh/sshconnect.c

Source
/* $OpenBSD: sshconnect.c,v 1.386 2026/09/16 00:29:44 djm Exp $ */
/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Code to connect to a remote host, and to perform the client side of the
 * login (authentication) dialog.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 */

#include "includes.h"

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>

#include <net/if.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <netdb.h>
#include <paths.h>
#include <pwd.h>
#include <poll.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <unistd.h>
#include <ifaddrs.h>

#include "xmalloc.h"
#include "hostfile.h"
#include "ssh.h"
#include "compat.h"
#include "packet.h"
#include "sshkey.h"
#include "sshconnect.h"
#include "log.h"
#include "match.h"
#include "misc.h"
#include "readconf.h"
#include "dns.h"
#include "monitor_fdpass.h"
#include "authfile.h"
#include "ssherr.h"
#include "authfd.h"
#include "kex.h"

struct sshkey *previous_host_key = NULL;

static int matching_host_key_dns = 0;

static pid_t proxy_command_pid = 0;

/* import */
extern int debug_flag;
extern Options options;
extern char *__progname;

static int show_other_keys(struct hostkeys *, struct sshkey *);
static void warn_changed_key(struct sshkey *);

void
ssh_conn_info_free(struct ssh_conn_info *cinfo)
{
  if (cinfo == NULL)
    return;
  free(cinfo->conn_hash_hex);
  free(cinfo->shorthost);
  free(cinfo->uidstr);
  free(cinfo->keyalias);
  free(cinfo->thishost);
  free(cinfo->host_arg);
  free(cinfo->portstr);
  free(cinfo->remhost);
  free(cinfo->remuser);
  free(cinfo->homedir);
  free(cinfo->locuser);
  free(cinfo->jmphost);
  freezero(cinfo, sizeof(*cinfo));
}

struct ssh_conn_info *
ssh_conn_info_dup(const struct ssh_conn_info *cinfo)
{
  struct ssh_conn_info *ret;

  if (cinfo == NULL)
    return NULL;
  ret = xcalloc(1, sizeof(*ret));
  ret->conn_hash_hex = xstrdup(cinfo->conn_hash_hex);
  ret->shorthost = xstrdup(cinfo->shorthost);
  ret->uidstr = xstrdup(cinfo->uidstr);
  ret->keyalias = xstrdup(cinfo->keyalias);
  ret->thishost = xstrdup(cinfo->thishost);
  ret->host_arg = xstrdup(cinfo->host_arg);
  ret->portstr = xstrdup(cinfo->portstr);
  ret->remhost = xstrdup(cinfo->remhost);
  ret->remuser = xstrdup(cinfo->remuser);
  ret->homedir = xstrdup(cinfo->homedir);
  ret->locuser = xstrdup(cinfo->locuser);
  ret->jmphost = xstrdup(cinfo->jmphost);
  return ret;
}

/* Expand a proxy command */
static char *
expand_proxy_command(const char *proxy_command, const char *user,
    const char *host, const char *host_arg, int port)
{
  char *tmp, *ret, strport[NI_MAXSERV];
  const char *keyalias = options.host_key_alias ?
      options.host_key_alias : host_arg;

  snprintf(strport, sizeof strport, "%d", port);
  xasprintf(&tmp, "exec %s", proxy_command);
  ret = percent_expand(tmp,
      "h", host,
      "k", keyalias,
      "n", host_arg,
      "p", strport,
      "r", options.user,
      (char *)NULL);
  free(tmp);
  return ret;
}

/*
 * Connect to the given ssh server using a proxy command that passes a
 * a connected fd back to us.
 */
static int
ssh_proxy_fdpass_connect(struct ssh *ssh, const char *host,
    const char *host_arg, u_short port, const char *proxy_command)
{
  char *command_string;
  int sp[2], sock;
  pid_t pid;
  char *shell;

  if ((shell = getenv("SHELL")) == NULL)
    shell = _PATH_BSHELL;

  if (socketpair(AF_UNIX, SOCK_STREAM, 0, sp) == -1)
    fatal("Could not create socketpair to communicate with "
        "proxy dialer: %.100s", strerror(errno));

  command_string = expand_proxy_command(proxy_command, options.user,
      host, host_arg, port);
  debug("Executing proxy dialer command: %.500s", command_string);

  /* Fork and execute the proxy command. */
  if ((pid = fork()) == 0) {
    char *argv[10];

    close(sp[1]);
    /* Redirect stdin and stdout. */
    if (sp[0] != 0) {
      if (dup2(sp[0], 0) == -1)
        perror("dup2 stdin");
    }
    if (sp[0] != 1) {
      if (dup2(sp[0], 1) == -1)
        perror("dup2 stdout");
    }
    if (sp[0] >= 2)
      close(sp[0]);

    /*
     * Stderr is left for non-ControlPersist connections is so
     * error messages may be printed on the user's terminal.
     */
    if (!debug_flag && options.control_path != NULL &&
        options.control_persist && stdfd_devnull(0, 0, 1) == -1)
      error_f("stdfd_devnull failed");

    argv[0] = shell;
    argv[1] = "-c";
    argv[2] = command_string;
    argv[3] = NULL;

    /*
     * Execute the proxy command.
     * Note that we gave up any extra privileges above.
     */
    execv(argv[0], argv);
    perror(argv[0]);
    exit(1);
  }
  /* Parent. */
  if (pid == -1)
    fatal("fork failed: %.100s", strerror(errno));
  close(sp[0]);
  free(command_string);

  if ((sock = mm_receive_fd(sp[1])) == -1)
    fatal("proxy dialer did not pass back a connection");
  close(sp[1]);

  while (waitpid(pid, NULL, 0) == -1)
    if (errno != EINTR)
      fatal("Couldn't wait for child: %s", strerror(errno));

  /* Set the connection file descriptors. */
  if (ssh_packet_set_connection(ssh, sock, sock) == NULL)
    return -1; /* ssh_packet_set_connection logs error */

  return 0;
}

/*
 * Connect to the given ssh server using a proxy command.
 */
static int
ssh_proxy_connect(struct ssh *ssh, const char *host, const char *host_arg,
    u_short port, const char *proxy_command)
{
  char *command_string;
  int pin[2], pout[2];
  pid_t pid;
  char *shell;

  if ((shell = getenv("SHELL")) == NULL || *shell == '\0')
    shell = _PATH_BSHELL;

  /* Create pipes for communicating with the proxy. */
  if (pipe(pin) == -1 || pipe(pout) == -1)
    fatal("Could not create pipes to communicate with the proxy: %.100s",
        strerror(errno));

  command_string = expand_proxy_command(proxy_command, options.user,
      host, host_arg, port);
  debug("Executing proxy command: %.500s", command_string);

  /* Fork and execute the proxy command. */
  if ((pid = fork()) == 0) {
    char *argv[10];

    /* Redirect stdin and stdout. */
    close(pin[1]);
    if (pin[0] != 0) {
      if (dup2(pin[0], 0) == -1)
        perror("dup2 stdin");
      close(pin[0]);
    }
    close(pout[0]);
    if (dup2(pout[1], 1) == -1)
      perror("dup2 stdout");
    /* Cannot be 1 because pin allocated two descriptors. */
    close(pout[1]);

    /*
     * Stderr is left for non-ControlPersist connections is so
     * error messages may be printed on the user's terminal.
     */
    if (!debug_flag && options.control_path != NULL &&
        options.control_persist && stdfd_devnull(0, 0, 1) == -1)
      error_f("stdfd_devnull failed");

    argv[0] = shell;
    argv[1] = "-c";
    argv[2] = command_string;
    argv[3] = NULL;

    /*
     * Execute the proxy command.  Note that we gave up any
     * extra privileges above.
     */
    ssh_signal(SIGPIPE, SIG_DFL);
    execv(argv[0], argv);
    perror(argv[0]);
    exit(1);
  }
  /* Parent. */
  if (pid == -1)
    fatal("fork failed: %.100s", strerror(errno));
  else
    proxy_command_pid = pid; /* save pid to clean up later */

  /* Close child side of the descriptors. */
  close(pin[0]);
  close(pout[1]);

  /* Free the command name. */
  free(command_string);

  /* Set the connection file descriptors. */
  if (ssh_packet_set_connection(ssh, pout[0], pin[1]) == NULL)
    return -1; /* ssh_packet_set_connection logs error */

  return 0;
}

void
ssh_kill_proxy_command(void)
{
  /*
   * Send SIGHUP to proxy command if used. We don't wait() in
   * case it hangs and instead rely on init to reap the child
   */
  if (proxy_command_pid > 1)
    kill(proxy_command_pid, SIGHUP);
}

#ifdef HAVE_IFADDRS_H
/*
 * Search a interface address list (returned from getifaddrs(3)) for an
 * address that matches the desired address family on the specified interface.
 * Returns 0 and fills in *resultp and *rlenp on success. Returns -1 on failure.
 */
static int
check_ifaddrs(const char *ifname, int af, const struct ifaddrs *ifaddrs,
    struct sockaddr_storage *resultp, socklen_t *rlenp)
{
  struct sockaddr_in6 *sa6;
  struct sockaddr_in *sa;
  struct in6_addr *v6addr;
  const struct ifaddrs *ifa;
  int allow_local;

  /*
   * Prefer addresses that are not loopback or linklocal, but use them
   * if nothing else matches.
   */
  for (allow_local = 0; allow_local < 2; allow_local++) {
    for (ifa = ifaddrs; ifa != NULL; ifa = ifa->ifa_next) {
      if (ifa->ifa_addr == NULL || ifa->ifa_name == NULL ||
          (ifa->ifa_flags & IFF_UP) == 0 ||
          ifa->ifa_addr->sa_family != af ||
          strcmp(ifa->ifa_name, options.bind_interface) != 0)
        continue;
      switch (ifa->ifa_addr->sa_family) {
      case AF_INET:
        sa = (struct sockaddr_in *)ifa->ifa_addr;
        if (!allow_local && sa->sin_addr.s_addr ==
            htonl(INADDR_LOOPBACK))
          continue;
        if (*rlenp < sizeof(struct sockaddr_in)) {
          error_f("v4 addr doesn't fit");
          return -1;
        }
        *rlenp = sizeof(struct sockaddr_in);
        memcpy(resultp, sa, *rlenp);
        return 0;
      case AF_INET6:
        sa6 = (struct sockaddr_in6 *)ifa->ifa_addr;
        v6addr = &sa6->sin6_addr;
        if (!allow_local &&
            (IN6_IS_ADDR_LINKLOCAL(v6addr) ||
            IN6_IS_ADDR_LOOPBACK(v6addr)))
          continue;
        if (*rlenp < sizeof(struct sockaddr_in6)) {
          error_f("v6 addr doesn't fit");
          return -1;
        }
        *rlenp = sizeof(struct sockaddr_in6);
        memcpy(resultp, sa6, *rlenp);
        return 0;
      }
    }
  }
  return -1;
}
#endif

/*
 * Creates a socket for use as the ssh connection.
 */
static int
ssh_create_socket(struct addrinfo *ai)
{
  int sock, r;
  struct sockaddr_storage bindaddr;
  socklen_t bindaddrlen = 0;
  struct addrinfo hints, *res = NULL;
#ifdef HAVE_IFADDRS_H
  struct ifaddrs *ifaddrs = NULL;
#endif
  char ntop[NI_MAXHOST];

  sock = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
  if (sock == -1) {
    error("socket: %s", strerror(errno));
    return -1;
  }
  (void)fcntl(sock, F_SETFD, FD_CLOEXEC);

  /* Use interactive QOS (if specified) until authentication completed */
  if (options.ip_qos_interactive != INT_MAX)
    set_sock_tos(sock, options.ip_qos_interactive);

  /* Bind the socket to an alternative local IP address */
  if (options.bind_address == NULL && options.bind_interface == NULL)
    return sock;

  if (options.bind_address != NULL) {
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = ai->ai_family;
    hints.ai_socktype = ai->ai_socktype;
    hints.ai_protocol = ai->ai_protocol;
    hints.ai_flags = AI_PASSIVE;
    if ((r = getaddrinfo(options.bind_address, NULL,
        &hints, &res)) != 0) {
      error("getaddrinfo: %s: %s", options.bind_address,
          ssh_gai_strerror(r));
      goto fail;
    }
    if (res == NULL) {
      error("getaddrinfo: no addrs");
      goto fail;
    }
    memcpy(&bindaddr, res->ai_addr, res->ai_addrlen);
    bindaddrlen = res->ai_addrlen;
  } else if (options.bind_interface != NULL) {
#ifdef HAVE_IFADDRS_H
    if ((r = getifaddrs(&ifaddrs)) != 0) {
      error("getifaddrs: %s: %s", options.bind_interface,
          strerror(errno));
      goto fail;
    }
    bindaddrlen = sizeof(bindaddr);
    if (check_ifaddrs(options.bind_interface, ai->ai_family,
        ifaddrs, &bindaddr, &bindaddrlen) != 0) {
      logit("getifaddrs: %s: no suitable addresses",
          options.bind_interface);
      goto fail;
    }
#else
    error("BindInterface not supported on this platform.");
#endif
  }
  if ((r = getnameinfo((struct sockaddr *)&bindaddr, bindaddrlen,
      ntop, sizeof(ntop), NULL, 0, NI_NUMERICHOST)) != 0) {
    error_f("getnameinfo failed: %s", ssh_gai_strerror(r));
    goto fail;
  }
  if (bind(sock, (struct sockaddr *)&bindaddr, bindaddrlen) != 0) {
    error("bind %s: %s", ntop, strerror(errno));
    goto fail;
  }
  debug_f("bound to %s", ntop);
  /* success */
  goto out;
fail:
  close(sock);
  sock = -1;
 out:
  if (res != NULL)
    freeaddrinfo(res);
#ifdef HAVE_IFADDRS_H
  if (ifaddrs != NULL)
    freeifaddrs(ifaddrs);
#endif
  return sock;
}

/*
 * Opens a TCP/IP connection to the remote server on the given host.
 * The address of the remote host will be returned in hostaddr.
 * If port is 0, the default port will be used.
 * Connection_attempts specifies the maximum number of tries (one per
 * second).  If proxy_command is non-NULL, it specifies the command (with %h
 * and %p substituted for host and port, respectively) to use to contact
 * the daemon.
 */
static int
ssh_connect_direct(struct ssh *ssh, const char *host, struct addrinfo *aitop,
    struct sockaddr_storage *hostaddr, u_short port, int connection_attempts,
    int *timeout_ms, int want_keepalive)
{
  int saved_timeout_ms = *timeout_ms;
  int oerrno, sock = -1, attempt;
  char ntop[NI_MAXHOST], strport[NI_MAXSERV];
  struct addrinfo *ai;

  debug3_f("entering");
  memset(ntop, 0, sizeof(ntop));
  memset(strport, 0, sizeof(strport));

  for (attempt = 0; attempt < connection_attempts; attempt++) {
    if (attempt > 0) {
      /* Sleep a moment before retrying. */
      sleep(1);
      debug("Trying again...");
    }
    /*
     * Loop through addresses for this host, and try each one in
     * sequence until the connection succeeds.
     */
    for (ai = aitop; ai; ai = ai->ai_next) {
      if (ai->ai_family != AF_INET &&
          ai->ai_family != AF_INET6) {
        errno = EAFNOSUPPORT;
        continue;
      }
      if (getnameinfo(ai->ai_addr, ai->ai_addrlen,
          ntop, sizeof(ntop), strport, sizeof(strport),
          NI_NUMERICHOST|NI_NUMERICSERV) != 0) {
        oerrno = errno;
        error_f("getnameinfo failed");
        errno = oerrno;
        continue;
      }
      if (options.address_family != AF_UNSPEC &&
          ai->ai_family != options.address_family) {
        debug2_f("skipping address [%s]:%s: "
            "wrong address family", ntop, strport);
        errno = EAFNOSUPPORT;
        continue;
      }

      debug("Connecting to %.200s [%.100s] port %s.",
        host, ntop, strport);

      /* Create a socket for connecting. */
      sock = ssh_create_socket(ai);
      if (sock < 0) {
        /* Any error is already output */
        errno = 0;
        continue;
      }

      *timeout_ms = saved_timeout_ms;
      if (timeout_connect(sock, ai->ai_addr, ai->ai_addrlen,
          timeout_ms) >= 0) {
        /* Successful connection. */
        memcpy(hostaddr, ai->ai_addr, ai->ai_addrlen);
        break;
      } else {
        oerrno = errno;
        debug("connect to address %s port %s: %s",
            ntop, strport, strerror(errno));
        close(sock);
        sock = -1;
        errno = oerrno;
      }
    }
    if (sock != -1)
      break; /* Successful connection. */
  }

  /* Return failure if we didn't get a successful connection. */
  if (sock == -1) {
    error("ssh: connect to host %s port %s: %s",
        host, strport, errno == 0 ? "failure" : strerror(errno));
    return -1;
  }

  debug("Connection established.");

  /* Set SO_KEEPALIVE if requested. */
  if (want_keepalive)
    set_keepalive(sock); /* logs errors */

  /* Set the connection. */
  if (ssh_packet_set_connection(ssh, sock, sock) == NULL)
    return -1; /* ssh_packet_set_connection logs error */

  return 0;
}

int
ssh_connect(struct ssh *ssh, const char *host, const char *host_arg,
    struct addrinfo *addrs, struct sockaddr_storage *hostaddr, u_short port,
    int connection_attempts, int *timeout_ms, int want_keepalive)
{
  int in, out;

  if (options.proxy_command == NULL) {
    return ssh_connect_direct(ssh, host, addrs, hostaddr, port,
        connection_attempts, timeout_ms, want_keepalive);
  } else if (strcmp(options.proxy_command, "-") == 0) {
    if ((in = dup(STDIN_FILENO)) == -1 ||
        (out = dup(STDOUT_FILENO)) == -1) {
      if (in >= 0)
        close(in);
      error_f("dup() in/out failed");
      return -1; /* ssh_packet_set_connection logs error */
    }
    if ((ssh_packet_set_connection(ssh, in, out)) == NULL)
      return -1; /* ssh_packet_set_connection logs error */
    return 0;
  } else if (options.proxy_use_fdpass) {
    return ssh_proxy_fdpass_connect(ssh, host, host_arg, port,
        options.proxy_command);
  }
  return ssh_proxy_connect(ssh, host, host_arg, port,
      options.proxy_command);
}

/* defaults to 'no' */
static int
confirm(const char *prompt, const char *fingerprint)
{
  const char *msg, *again = "Please type 'yes' or 'no': ";
  const char *again_fp = "Please type 'yes', 'no' or the fingerprint: ";
  char *p, *cp;
  int ret = -1;

  if (options.batch_mode)
    return 0;
  for (msg = prompt;;msg = fingerprint ? again_fp : again) {
    cp = p = read_passphrase(msg, RP_ECHO);
    if (p == NULL)
      return 0;
    p += strspn(p, " \t"); /* skip leading whitespace */
    p[strcspn(p, " \t\n")] = '\0'; /* remove trailing whitespace */
    if (p[0] == '\0' || strcasecmp(p, "no") == 0)
      ret = 0;
    else if (strcasecmp(p, "yes") == 0 || (fingerprint != NULL &&
        strcmp(p, fingerprint) == 0))
      ret = 1;
    free(cp);
    if (ret != -1)
      return ret;
  }
}

static int
sockaddr_is_local(struct sockaddr *hostaddr)
{
  switch (hostaddr->sa_family) {
  case AF_INET:
    return (ntohl(((struct sockaddr_in *)hostaddr)->
        sin_addr.s_addr) >> 24) == IN_LOOPBACKNET;
  case AF_INET6:
    return IN6_IS_ADDR_LOOPBACK(
        &(((struct sockaddr_in6 *)hostaddr)->sin6_addr));
  default:
    return 0;
  }
}

/*
 * Prepare the hostname and ip address strings that are used to lookup
 * host keys in known_hosts files. These may have a port number appended.
 */
void
get_hostfile_hostname_ipaddr(char *hostname, struct sockaddr *hostaddr,
    u_short port, char **hostfile_hostname, char **hostfile_ipaddr)
{
  char ntop[NI_MAXHOST];
  socklen_t addrlen;

  switch (hostaddr == NULL ? -1 : hostaddr->sa_family) {
  case -1:
    addrlen = 0;
    break;
  case AF_INET:
    addrlen = sizeof(struct sockaddr_in);
    break;
  case AF_INET6:
    addrlen = sizeof(struct sockaddr_in6);
    break;
  default:
    addrlen = sizeof(struct sockaddr);
    break;
  }

  /*
   * We don't have the remote ip-address for connections
   * using a proxy command
   */
  if (hostfile_ipaddr != NULL) {
    if (options.proxy_command == NULL) {
      if (getnameinfo(hostaddr, addrlen,
          ntop, sizeof(ntop), NULL, 0, NI_NUMERICHOST) != 0)
        fatal_f("getnameinfo failed");
      *hostfile_ipaddr = put_host_port(ntop, port);
    } else {
      *hostfile_ipaddr = xstrdup("<no hostip for proxy "
          "command>");
    }
  }

  /*
   * Allow the user to record the key under a different name or
   * differentiate a non-standard port.  This is useful for ssh
   * tunneling over forwarded connections or if you run multiple
   * sshd's on different ports on the same machine.
   */
  if (hostfile_hostname != NULL) {
    if (options.host_key_alias != NULL) {
      *hostfile_hostname = xstrdup(options.host_key_alias);
      debug("using hostkeyalias: %s", *hostfile_hostname);
    } else {
      *hostfile_hostname = put_host_port(hostname, port);
    }
  }
}

/* returns non-zero if path appears in hostfiles, or 0 if not. */
static int
path_in_hostfiles(const char *path, char **hostfiles, u_int num_hostfiles)
{
  u_int i;

  for (i = 0; i < num_hostfiles; i++) {
    if (strcmp(path, hostfiles[i]) == 0)
      return 1;
  }
  return 0;
}

struct find_by_key_ctx {
  const char *host, *ip;
  const struct sshkey *key;
  char **names;
  u_int nnames;
};

/* Try to replace home directory prefix (per $HOME) with a ~/ sequence */
static char *
try_tilde_unexpand(const char *path)
{
  char *home, *ret = NULL;
  size_t l;

  if (*path != '/')
    return xstrdup(path);
  if ((home = getenv("HOME")) == NULL || (l = strlen(home)) == 0)
    return xstrdup(path);
  if (strncmp(path, home, l) != 0)
    return xstrdup(path);
  /*
   * ensure we have matched on a path boundary: either the $HOME that
   * we just compared ends with a '/' or the next character of the path
   * must be a '/'.
   */
  if (home[l - 1] != '/' && path[l] != '/')
    return xstrdup(path);
  if (path[l] == '/')
    l++;
  xasprintf(&ret, "~/%s", path + l);
  return ret;
}

/*
 * Returns non-zero if the key is accepted by HostkeyAlgorithms.
 * Made slightly less trivial by the multiple RSA signature algorithm names.
 */
int
hostkey_accepted_by_hostkeyalgs(const struct sshkey *key)
{
  const char *ktype = sshkey_ssh_name(key);
  const char *hostkeyalgs = options.hostkeyalgorithms;

  if (key->type == KEY_UNSPEC)
    return 0;
  if (key->type == KEY_RSA &&
      (match_pattern_list("rsa-sha2-256", hostkeyalgs, 0) == 1 ||
      match_pattern_list("rsa-sha2-512", hostkeyalgs, 0) == 1))
    return 1;
  if (key->type == KEY_RSA_CERT &&
      (match_pattern_list("rsa-sha2-512-cert-v01@openssh.com", hostkeyalgs, 0) == 1 ||
      match_pattern_list("rsa-sha2-256-cert-v01@openssh.com", hostkeyalgs, 0) == 1))
    return 1;
  return match_pattern_list(ktype, hostkeyalgs, 0) == 1;
}

static int
hostkeys_find_by_key_cb(struct hostkey_foreach_line *l, void *_ctx)
{
  struct find_by_key_ctx *ctx = (struct find_by_key_ctx *)_ctx;
  char *path;

  /* we are looking for keys with names that *do not* match */
  if ((l->match & HKF_MATCH_HOST) != 0)
    return 0;
  /* not interested in marker lines */
  if (l->marker != MRK_NONE)
    return 0;
  /* we are only interested in exact key matches */
  if (l->key == NULL || !sshkey_equal(ctx->key, l->key))
    return 0;
  path = try_tilde_unexpand(l->path);
  debug_f("found matching key in %s:%lu", path, l->linenum);
  ctx->names = xrecallocarray(ctx->names,
      ctx->nnames, ctx->nnames + 1, sizeof(*ctx->names));
  xasprintf(&ctx->names[ctx->nnames], "%s:%lu: %s", path, l->linenum,
      strncmp(l->hosts, HASH_MAGIC, strlen(HASH_MAGIC)) == 0 ?
      "[hashed name]" : l->hosts);
  ctx->nnames++;
  free(path);
  return 0;
}

static int
hostkeys_find_by_key_hostfile(const char *file, const char *which,
    struct find_by_key_ctx *ctx)
{
  int r;

  debug3_f("trying %s hostfile \"%s\"", which, file);
  if ((r = hostkeys_foreach(file, hostkeys_find_by_key_cb, ctx,
      ctx->host, ctx->ip, HKF_WANT_PARSE_KEY, 0)) != 0) {
    if (r == SSH_ERR_SYSTEM_ERROR && errno == ENOENT) {
      debug_f("hostkeys file %s does not exist", file);
      return 0;
    }
    error_fr(r, "hostkeys_foreach failed for %s", file);
    return r;
  }
  return 0;
}

/*
 * Find 'key' in known hosts file(s) that do not match host/ip.
 * Used to display also-known-as information for previously-unseen hostkeys.
 */
static void
hostkeys_find_by_key(const char *host, const char *ip, const struct sshkey *key,
    char **user_hostfiles, u_int num_user_hostfiles,
    char **system_hostfiles, u_int num_system_hostfiles,
    char ***names, u_int *nnames)
{
  struct find_by_key_ctx ctx = {NULL, NULL, NULL, NULL, 0};
  u_int i;

  *names = NULL;
  *nnames = 0;

  if (key == NULL || sshkey_is_cert(key))
    return;

  ctx.host = host;
  ctx.ip = ip;
  ctx.key = key;

  for (i = 0; i < num_user_hostfiles; i++) {
    if (hostkeys_find_by_key_hostfile(user_hostfiles[i],
        "user", &ctx) != 0)
      goto fail;
  }
  for (i = 0; i < num_system_hostfiles; i++) {
    if (hostkeys_find_by_key_hostfile(system_hostfiles[i],
        "system", &ctx) != 0)
      goto fail;
  }
  /* success */
  *names = ctx.names;
  *nnames = ctx.nnames;
  ctx.names = NULL;
  ctx.nnames = 0;
  return;
 fail:
  for (i = 0; i < ctx.nnames; i++)
    free(ctx.names[i]);
  free(ctx.names);
}

#define MAX_OTHER_NAMES 8 /* Maximum number of names to list */
static char *
other_hostkeys_message(const char *host, const char *ip,
    const struct sshkey *key,
    char **user_hostfiles, u_int num_user_hostfiles,
    char **system_hostfiles, u_int num_system_hostfiles)
{
  char *ret = NULL, **othernames = NULL;
  u_int i, n, num_othernames = 0;

  hostkeys_find_by_key(host, ip, key,
      user_hostfiles, num_user_hostfiles,
      system_hostfiles, num_system_hostfiles,
      &othernames, &num_othernames);
  if (num_othernames == 0)
    return xstrdup("This key is not known by any other names.");

  xasprintf(&ret, "This host key is known by the following other "
      "names/addresses:");

  n = num_othernames;
  if (n > MAX_OTHER_NAMES)
    n = MAX_OTHER_NAMES;
  for (i = 0; i < n; i++) {
    xextendf(&ret, "\n", "    %s", othernames[i]);
  }
  if (n < num_othernames) {
    xextendf(&ret, "\n", "    (%d additional names omitted)",
        num_othernames - n);
  }
  for (i = 0; i < num_othernames; i++)
    free(othernames[i]);
  free(othernames);
  return ret;
}

void
load_hostkeys_command(struct hostkeys *hostkeys, const char *command_template,
    const char *invocation, const struct ssh_conn_info *cinfo,
    const struct sshkey *host_key, const char *hostfile_hostname)
{
  int r, i, ac = 0;
  char *key_fp = NULL, *keytext = NULL, *tmp;
  char *command = NULL, *tag = NULL, **av = NULL;
  FILE *f = NULL;
  pid_t pid;
  void (*osigchld)(int);

  xasprintf(&tag, "KnownHostsCommand-%s", invocation);

  if (host_key != NULL) {
    if ((key_fp = sshkey_fingerprint(host_key,
        options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
      fatal_f("sshkey_fingerprint failed");
    if ((r = sshkey_to_base64(host_key, &keytext)) != 0)
      fatal_fr(r, "sshkey_to_base64 failed");
  }
  /*
   * NB. all returns later this function should go via "out" to
   * ensure the original SIGCHLD handler is restored properly.
   */
  osigchld = ssh_signal(SIGCHLD, SIG_DFL);

  /* Turn the command into an argument vector */
  if (argv_split(command_template, &ac, &av, 0) != 0) {
    error("%s \"%s\" contains invalid quotes", tag,
        command_template);
    goto out;
  }
  if (ac == 0) {
    error("%s \"%s\" yielded no arguments", tag,
        command_template);
    goto out;
  }
  for (i = 1; i < ac; i++) {
    tmp = percent_dollar_expand(av[i],
        DEFAULT_CLIENT_PERCENT_EXPAND_ARGS(cinfo),
        "H", hostfile_hostname,
        "I", invocation,
        "t", host_key == NULL ? "NONE" : sshkey_ssh_name(host_key),
        "f", key_fp == NULL ? "NONE" : key_fp,
        "K", keytext == NULL ? "NONE" : keytext,
        (char *)NULL);
    if (tmp == NULL)
      fatal_f("percent_expand failed");
    free(av[i]);
    av[i] = tmp;
  }
  /* Prepare a printable command for logs, etc. */
  command = argv_assemble(ac, av);

  if ((pid = subprocess(tag, command, ac, av, &f,
      SSH_SUBPROCESS_STDOUT_CAPTURE|SSH_SUBPROCESS_UNSAFE_PATH|
      SSH_SUBPROCESS_PRESERVE_ENV, NULL, NULL, NULL)) == 0)
    goto out;

  load_hostkeys_file(hostkeys, hostfile_hostname, tag, f, 1);

  if (exited_cleanly(pid, tag, command, 0) != 0)
    fatal("KnownHostsCommand failed");

 out:
  if (f != NULL)
    fclose(f);
  ssh_signal(SIGCHLD, osigchld);
  for (i = 0; i < ac; i++)
    free(av[i]);
  free(av);
  free(tag);
  free(command);
  free(key_fp);
  free(keytext);
}

/*
 * check whether the supplied host key is valid, return -1 if the key
 * is not valid. user_hostfile[0] will not be updated if 'readonly' is true.
 */
#define RDRW  0
#define RDONLY  1
#define ROQUIET 2
static int
check_host_key(char *hostname, const struct ssh_conn_info *cinfo,
    struct sockaddr *hostaddr, u_short port,
    struct sshkey *host_key, int readonly, int clobber_port,
    char **user_hostfiles, u_int num_user_hostfiles,
    char **system_hostfiles, u_int num_system_hostfiles,
    const char *hostfile_command)
{
  HostStatus host_status = -1, ip_status = -1;
  struct sshkey *raw_key = NULL;
  char *ip = NULL, *host = NULL;
  char hostline[1000], *hostp, *fp, *ra;
  char msg[1024];
  const char *type, *fail_reason = NULL;
  const struct hostkey_entry *host_found = NULL, *ip_found = NULL;
  int len, cancelled_forwarding = 0, confirmed;
  int local = sockaddr_is_local(hostaddr);
  int r, want_cert = sshkey_is_cert(host_key), host_ip_differ = 0;
  int hostkey_trusted = 0; /* Known or explicitly accepted by user */
  struct hostkeys *host_hostkeys, *ip_hostkeys;
  u_int i;

  /*
   * Force accepting of the host key for loopback/localhost. The
   * problem is that if the home directory is NFS-mounted to multiple
   * machines, localhost will refer to a different machine in each of
   * them, and the user will get bogus HOST_CHANGED warnings.  This
   * essentially disables host authentication for localhost; however,
   * this is probably not a real problem.
   */
  if (options.no_host_authentication_for_localhost == 1 && local &&
      options.host_key_alias == NULL) {
    debug("Forcing accepting of host key for "
        "loopback/localhost.");
    options.update_hostkeys = 0;
    return 0;
  }

  /*
   * Don't ever try to write an invalid name to a known hosts file.
   * Note: do this before get_hostfile_hostname_ipaddr() to catch
   * '[' or ']' in the name before they are added.
   */
  if (strcspn(hostname, "@?*#[]|'\'\"\\") != strlen(hostname)) {
    debug_f("invalid hostname \"%s\"; will not record: %s",
        hostname, fail_reason);
    readonly = RDONLY;
  }

  /*
   * Prepare the hostname and address strings used for hostkey lookup.
   * In some cases, these will have a port number appended.
   */
  get_hostfile_hostname_ipaddr(hostname, hostaddr,
      clobber_port ? 0 : port, &host, &ip);

  /*
   * Turn off check_host_ip if the connection is to localhost, via proxy
   * command or if we don't have a hostname to compare with
   */
  if (options.check_host_ip && (local ||
      strcmp(hostname, ip) == 0 || options.proxy_command != NULL))
    options.check_host_ip = 0;

  host_hostkeys = init_hostkeys();
  for (i = 0; i < num_user_hostfiles; i++)
    load_hostkeys(host_hostkeys, host, user_hostfiles[i], 0);
  for (i = 0; i < num_system_hostfiles; i++)
    load_hostkeys(host_hostkeys, host, system_hostfiles[i], 0);
  if (hostfile_command != NULL && !clobber_port) {
    load_hostkeys_command(host_hostkeys, hostfile_command,
        "HOSTNAME", cinfo, host_key, host);
  }

  ip_hostkeys = NULL;
  if (!want_cert && options.check_host_ip) {
    ip_hostkeys = init_hostkeys();
    for (i = 0; i < num_user_hostfiles; i++)
      load_hostkeys(ip_hostkeys, ip, user_hostfiles[i], 0);
    for (i = 0; i < num_system_hostfiles; i++)
      load_hostkeys(ip_hostkeys, ip, system_hostfiles[i], 0);
    if (hostfile_command != NULL && !clobber_port) {
      load_hostkeys_command(ip_hostkeys, hostfile_command,
          "ADDRESS", cinfo, host_key, ip);
    }
  }

 retry:
  if (!hostkey_accepted_by_hostkeyalgs(host_key)) {
    error("host key %s not permitted by HostkeyAlgorithms",
        sshkey_ssh_name(host_key));
    goto fail;
  }

  /* Reload these as they may have changed on cert->key downgrade */
  want_cert = sshkey_is_cert(host_key);
  type = sshkey_type(host_key);

  /*
   * Check if the host key is present in the user's list of known
   * hosts or in the systemwide list.
   */
  host_status = check_key_in_hostkeys(host_hostkeys, host_key,
      &host_found);

  /*
   * If there are no hostfiles, or if the hostkey was found via
   * KnownHostsCommand, then don't try to touch the disk.
   */
  if (!readonly && (num_user_hostfiles == 0 ||
      (host_found != NULL && host_found->note != 0)))
    readonly = RDONLY;

  /*
   * Also perform check for the ip address, skip the check if we are
   * localhost, looking for a certificate, or the hostname was an ip
   * address to begin with.
   */
  if (!want_cert && ip_hostkeys != NULL) {
    ip_status = check_key_in_hostkeys(ip_hostkeys, host_key,
        &ip_found);
    if (host_status == HOST_CHANGED &&
        (ip_status != HOST_CHANGED ||
        (ip_found != NULL &&
        !sshkey_equal(ip_found->key, host_found->key))))
      host_ip_differ = 1;
  } else
    ip_status = host_status;

  switch (host_status) {
  case HOST_OK:
    /* The host is known and the key matches. */
    debug("Host '%.200s' is known and matches the %s host %s.",
        host, type, want_cert ? "certificate" : "key");
    debug("Found %s in %s:%lu", want_cert ? "CA key" : "key",
        host_found->file, host_found->line);
    if (want_cert) {
      if (sshkey_cert_check_host(host_key,
          options.host_key_alias == NULL ?
          hostname : options.host_key_alias,
          options.ca_sign_algorithms, &fail_reason) != 0) {
        error("%s", fail_reason);
        goto fail;
      }
      /*
       * Do not attempt hostkey update if a certificate was
       * successfully matched.
       */
      if (options.update_hostkeys != 0) {
        options.update_hostkeys = 0;
        debug3_f("certificate host key in use; "
            "disabling UpdateHostkeys");
      }
    }
    /* Turn off UpdateHostkeys if key was in system known_hosts */
    if (options.update_hostkeys != 0 &&
        (path_in_hostfiles(host_found->file,
        system_hostfiles, num_system_hostfiles) ||
        (ip_status == HOST_OK && ip_found != NULL &&
        path_in_hostfiles(ip_found->file,
        system_hostfiles, num_system_hostfiles)))) {
      options.update_hostkeys = 0;
      debug3_f("host key found in GlobalKnownHostsFile; "
          "disabling UpdateHostkeys");
    }
    if (options.update_hostkeys != 0 && host_found->note) {
      options.update_hostkeys = 0;
      debug3_f("host key found via KnownHostsCommand; "
          "disabling UpdateHostkeys");
    }
    if (options.check_host_ip && ip_status == HOST_NEW) {
      if (readonly || want_cert)
        logit("%s host key for IP address "
            "'%.128s' not in list of known hosts.",
            type, ip);
      else if (!add_host_to_hostfile(user_hostfiles[0], ip,
          host_key, options.hash_known_hosts))
        logit("Failed to add the %s host key for IP "
            "address '%.128s' to the list of known "
            "hosts (%.500s).", type, ip,
            user_hostfiles[0]);
      else
        logit("Warning: Permanently added the %s host "
            "key for IP address '%.128s' to the list "
            "of known hosts.", type, ip);
    } else if (options.visual_host_key) {
      fp = sshkey_fingerprint(host_key,
          options.fingerprint_hash, SSH_FP_DEFAULT);
      ra = sshkey_fingerprint(host_key,
          options.fingerprint_hash, SSH_FP_RANDOMART);
      if (fp == NULL || ra == NULL)
        fatal_f("sshkey_fingerprint failed");
      logit("Host key fingerprint is: %s\n%s", fp, ra);
      free(ra);
      free(fp);
    }
    hostkey_trusted = 1;
    break;
  case HOST_NEW:
    if (options.host_key_alias == NULL && port != 0 &&
        port != SSH_DEFAULT_PORT && !clobber_port) {
      debug("checking without port identifier");
      if (check_host_key(hostname, cinfo, hostaddr, 0,
          host_key, ROQUIET, 1,
          user_hostfiles, num_user_hostfiles,
          system_hostfiles, num_system_hostfiles,
          hostfile_command) == 0) {
        debug("found matching key w/out port");
        break;
      }
    }
    if (readonly || want_cert)
      goto fail;
    /* The host is new. */
    if (options.strict_host_key_checking ==
        SSH_STRICT_HOSTKEY_YES) {
      /*
       * User has requested strict host key checking.  We
       * will not add the host key automatically.  The only
       * alternative left is to abort.
       */
      error("No %s host key is known for %.200s and you "
          "have requested strict checking.", type, host);
      goto fail;
    } else if (options.strict_host_key_checking ==
        SSH_STRICT_HOSTKEY_ASK) {
      char *msg1 = NULL, *msg2 = NULL;

      xasprintf(&msg1, "The authenticity of host "
          "'%.200s (%s)' can't be established", host, ip);

      if (show_other_keys(host_hostkeys, host_key)) {
        xextendf(&msg1, "\n", "but keys of different "
            "type are already known for this host.");
      } else
        xextendf(&msg1, "", ".");

      fp = sshkey_fingerprint(host_key,
          options.fingerprint_hash, SSH_FP_DEFAULT);
      ra = sshkey_fingerprint(host_key,
          options.fingerprint_hash, SSH_FP_RANDOMART);
      if (fp == NULL || ra == NULL)
        fatal_f("sshkey_fingerprint failed");
      xextendf(&msg1, "\n", "%s key fingerprint is: %s",
          type, fp);
      if (options.visual_host_key)
        xextendf(&msg1, "\n", "%s", ra);
      if (options.verify_host_key_dns) {
        xextendf(&msg1, "\n",
            "%s host key fingerprint found in DNS.",
            matching_host_key_dns ?
            "Matching" : "No matching");
      }
      /* msg2 informs for other names matching this key */
      if ((msg2 = other_hostkeys_message(host, ip, host_key,
          user_hostfiles, num_user_hostfiles,
          system_hostfiles, num_system_hostfiles)) != NULL)
        xextendf(&msg1, "\n", "%s", msg2);

      xextendf(&msg1, "\n",
          "Are you sure you want to continue connecting "
          "(yes/no/[fingerprint])? ");

      confirmed = confirm(msg1, fp);
      free(ra);
      free(fp);
      free(msg1);
      free(msg2);
      if (!confirmed)
        goto fail;
      hostkey_trusted = 1; /* user explicitly confirmed */
    }
    /*
     * If in "new" or "off" strict mode, add the key automatically
     * to the local known_hosts file.
     */
    if (options.check_host_ip && ip_status == HOST_NEW) {
      snprintf(hostline, sizeof(hostline), "%s,%s", host, ip);
      hostp = hostline;
      if (options.hash_known_hosts) {
        /* Add hash of host and IP separately */
        r = add_host_to_hostfile(user_hostfiles[0],
            host, host_key, options.hash_known_hosts) &&
            add_host_to_hostfile(user_hostfiles[0], ip,
            host_key, options.hash_known_hosts);
      } else {
        /* Add unhashed "host,ip" */
        r = add_host_to_hostfile(user_hostfiles[0],
            hostline, host_key,
            options.hash_known_hosts);
      }
    } else {
      r = add_host_to_hostfile(user_hostfiles[0], host,
          host_key, options.hash_known_hosts);
      hostp = host;
    }

    if (!r)
      logit("Failed to add the host to the list of known "
          "hosts (%.500s).", user_hostfiles[0]);
    else
      logit("Warning: Permanently added '%.200s' (%s) to the "
          "list of known hosts.", hostp, type);
    break;
  case HOST_REVOKED:
    error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
    error("@       WARNING: REVOKED HOST KEY DETECTED!               @");
    error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
    error("The %s host key for %s is marked as revoked.", type, host);
    error("This could mean that a stolen key is being used to");
    error("impersonate this host.");

    /*
     * If strict host key checking is in use, the user will have
     * to edit the key manually and we can only abort.
     */
    if (options.strict_host_key_checking !=
        SSH_STRICT_HOSTKEY_OFF) {
      error("%s host key for %.200s was revoked and you have "
          "requested strict checking.", type, host);
      goto fail;
    }
    goto continue_unsafe;

  case HOST_CHANGED:
    if (want_cert) {
      /*
       * This is only a debug() since it is valid to have
       * CAs with wildcard DNS matches that don't match
       * all hosts that one might visit.
       */
      debug("Host certificate authority does not "
          "match %s in %s:%lu", CA_MARKER,
          host_found->file, host_found->line);
      goto fail;
    }
    if (readonly == ROQUIET)
      goto fail;
    if (options.check_host_ip && host_ip_differ) {
      char *key_msg;
      if (ip_status == HOST_NEW)
        key_msg = "is unknown";
      else if (ip_status == HOST_OK)
        key_msg = "is unchanged";
      else
        key_msg = "has a different value";
      error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
      error("@       WARNING: POSSIBLE DNS SPOOFING DETECTED!          @");
      error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
      error("The %s host key for %s has changed,", type, host);
      error("and the key for the corresponding IP address %s", ip);
      error("%s. This could either mean that", key_msg);
      error("DNS SPOOFING is happening or the IP address for the host");
      error("and its host key have changed at the same time.");
      if (ip_status != HOST_NEW)
        error("Offending key for IP in %s:%lu",
            ip_found->file, ip_found->line);
    }
    /* The host key has changed. */
    warn_changed_key(host_key);
    if (num_user_hostfiles > 0 || num_system_hostfiles > 0) {
      error("Add correct host key in %.100s to get rid "
          "of this message.", num_user_hostfiles > 0 ?
          user_hostfiles[0] : system_hostfiles[0]);
    }
    error("Offending %s key in %s:%lu",
        sshkey_type(host_found->key),
        host_found->file, host_found->line);

    /*
     * If strict host key checking is in use, the user will have
     * to edit the key manually and we can only abort.
     */
    if (options.strict_host_key_checking !=
        SSH_STRICT_HOSTKEY_OFF) {
      error("Host key for %.200s has changed and you have "
          "requested strict checking.", host);
      goto fail;
    }

 continue_unsafe:
    /*
     * If strict host key checking has not been requested, allow
     * the connection but without MITM-able authentication or
     * forwarding.
     */
    if (options.password_authentication) {
      error("Password authentication is disabled to avoid "
          "man-in-the-middle attacks.");
      options.password_authentication = 0;
      cancelled_forwarding = 1;
    }
    if (options.kbd_interactive_authentication) {
      error("Keyboard-interactive authentication is disabled"
          " to avoid man-in-the-middle attacks.");
      options.kbd_interactive_authentication = 0;
      cancelled_forwarding = 1;
    }
    if (options.forward_agent) {
      error("Agent forwarding is disabled to avoid "
          "man-in-the-middle attacks.");
      options.forward_agent = 0;
      cancelled_forwarding = 1;
    }
    if (options.forward_x11) {
      error("X11 forwarding is disabled to avoid "
          "man-in-the-middle attacks.");
      options.forward_x11 = 0;
      cancelled_forwarding = 1;
    }
    if (options.num_local_forwards > 0 ||
        options.num_remote_forwards > 0) {
      error("Port forwarding is disabled to avoid "
          "man-in-the-middle attacks.");
      options.num_local_forwards =
          options.num_remote_forwards = 0;
      cancelled_forwarding = 1;
    }
    if (options.tun_open != SSH_TUNMODE_NO) {
      error("Tunnel forwarding is disabled to avoid "
          "man-in-the-middle attacks.");
      options.tun_open = SSH_TUNMODE_NO;
      cancelled_forwarding = 1;
    }
    if (options.update_hostkeys != 0) {
      error("UpdateHostkeys is disabled because the host "
          "key is not trusted.");
      options.update_hostkeys = 0;
    }
    if (options.exit_on_forward_failure && cancelled_forwarding)
      fatal("Error: forwarding disabled due to host key "
          "check failure");

    /*
     * XXX Should permit the user to change to use the new id.
     * This could be done by converting the host key to an
     * identifying sentence, tell that the host identifies itself
     * by that sentence, and ask the user if they wish to
     * accept the authentication.
     */
    break;
  case HOST_FOUND:
    fatal("internal error");
    break;
  }

  if (options.check_host_ip && host_status != HOST_CHANGED &&
      ip_status == HOST_CHANGED) {
    snprintf(msg, sizeof(msg),
        "Warning: the %s host key for '%.200s' "
        "differs from the key for the IP address '%.128s'"
        "\nOffending key for IP in %s:%lu",
        type, host, ip, ip_found->file, ip_found->line);
    if (host_status == HOST_OK) {
      len = strlen(msg);
      snprintf(msg + len, sizeof(msg) - len,
          "\nMatching host key in %s:%lu",
          host_found->file, host_found->line);
    }
    if (options.strict_host_key_checking ==
        SSH_STRICT_HOSTKEY_ASK) {
      strlcat(msg, "\nAre you sure you want "
          "to continue connecting (yes/no)? ", sizeof(msg));
      if (!confirm(msg, NULL))
        goto fail;
    } else if (options.strict_host_key_checking !=
        SSH_STRICT_HOSTKEY_OFF) {
      logit("%s", msg);
      error("Exiting, you have requested strict checking.");
      goto fail;
    } else {
      logit("%s", msg);
    }
  }

  if (!hostkey_trusted && options.update_hostkeys) {
    debug_f("hostkey not known or explicitly trusted: "
        "disabling UpdateHostkeys");
    options.update_hostkeys = 0;
  }

  sshkey_free(raw_key);
  free(ip);
  free(host);
  if (host_hostkeys != NULL)
    free_hostkeys(host_hostkeys);
  if (ip_hostkeys != NULL)
    free_hostkeys(ip_hostkeys);
  return 0;

fail:
  if (want_cert && host_status != HOST_REVOKED) {
    /*
     * No matching certificate. Downgrade cert to raw key and
     * search normally.
     */
    debug("No matching CA found. Retry with plain key");
    if ((r = sshkey_from_private(host_key, &raw_key)) != 0)
      fatal_fr(r, "decode key");
    if ((r = sshkey_drop_cert(raw_key)) != 0)
      fatal_r(r, "Couldn't drop certificate");
    host_key = raw_key;
    goto retry;
  }
  sshkey_free(raw_key);
  free(ip);
  free(host);
  if (host_hostkeys != NULL)
    free_hostkeys(host_hostkeys);
  if (ip_hostkeys != NULL)
    free_hostkeys(ip_hostkeys);
  return -1;
}

/* returns 0 if key verifies or -1 if key does NOT verify */
int
verify_host_key(char *host, struct sockaddr *hostaddr, struct sshkey *host_key,
    const struct ssh_conn_info *cinfo)
{
  u_int i;
  int r = -1, flags = 0;
  char valid[64], *fp = NULL, *cafp = NULL;
  struct sshkey *plain = NULL;

  if ((fp = sshkey_fingerprint(host_key,
      options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL) {
    error_fr(r, "fingerprint host key");
    r = -1;
    goto out;
  }

  if (sshkey_is_cert(host_key)) {
    if ((cafp = sshkey_fingerprint(host_key->cert->signature_key,
        options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL) {
      error_fr(r, "fingerprint CA key");
      r = -1;
      goto out;
    }
    sshkey_format_cert_validity(host_key->cert,
        valid, sizeof(valid));
    debug("Server host certificate: %s %s, serial %llu "
        "ID \"%s\" CA %s %s valid %s",
        sshkey_ssh_name(host_key), fp,
        (unsigned long long)host_key->cert->serial,
        host_key->cert->key_id,
        sshkey_ssh_name(host_key->cert->signature_key), cafp,
        valid);
    for (i = 0; i < host_key->cert->nprincipals; i++) {
      debug2("Server host certificate hostname: %s",
          host_key->cert->principals[i]);
    }
  } else {
    debug("Server host key: %s %s", sshkey_ssh_name(host_key), fp);
  }

  if (sshkey_equal(previous_host_key, host_key)) {
    debug2_f("server host key %s %s matches cached key",
        sshkey_type(host_key), fp);
    r = 0;
    goto out;
  }

  /* Check in RevokedHostKeys files if specified */
  for (i = 0; i < options.num_revoked_host_keys; i++) {
    r = sshkey_check_revoked(host_key,
        options.revoked_host_keys[i]);
    switch (r) {
    case 0:
      break; /* not revoked */
    case SSH_ERR_KEY_REVOKED:
      error("Host key %s %s revoked by file %s",
          sshkey_type(host_key), fp,
          options.revoked_host_keys[i]);
      r = -1;
      goto out;
    default:
      error_r(r, "Error checking host key %s %s in "
          "revoked keys file %s", sshkey_type(host_key),
          fp, options.revoked_host_keys[i]);
      r = -1;
      goto out;
    }
  }

  if (options.verify_host_key_dns) {
    /*
     * XXX certs are not yet supported for DNS, so downgrade
     * them and try the plain key.
     */
    if ((r = sshkey_from_private(host_key, &plain)) != 0)
      goto out;
    if (sshkey_is_cert(plain))
      sshkey_drop_cert(plain);
    if (verify_host_key_dns(host, hostaddr, plain, &flags) == 0) {
      if (flags & DNS_VERIFY_FOUND) {
        if (options.verify_host_key_dns == 1 &&
            flags & DNS_VERIFY_MATCH &&
            flags & DNS_VERIFY_SECURE) {
          r = 0;
          goto out;
        }
        if (flags & DNS_VERIFY_MATCH) {
          matching_host_key_dns = 1;
        } else {
          warn_changed_key(plain);
          error("Update the SSHFP RR in DNS "
              "with the new host key to get rid "
              "of this message.");
        }
      }
    }
  }
  r = check_host_key(host, cinfo, hostaddr, options.port, host_key,
      RDRW, 0, options.user_hostfiles, options.num_user_hostfiles,
      options.system_hostfiles, options.num_system_hostfiles,
      options.known_hosts_command);

out:
  sshkey_free(plain);
  free(fp);
  free(cafp);
  if (r == 0 && host_key != NULL) {
    sshkey_free(previous_host_key);
    r = sshkey_from_private(host_key, &previous_host_key);
  }

  return r;
}

static void
warn_nonpq_kex(void)
{
  logit("** WARNING: connection is not using a post-quantum key exchange algorithm.");
  logit("** This session may be vulnerable to \"store now, decrypt later\" attacks.");
  logit("** The server may need to be upgraded. See https://openssh.com/pq.html");
}

/*
 * Starts a dialog with the server, and authenticates the current user on the
 * server.  This does not need any extra privileges.  The basic connection
 * to the server must already have been established before this is called.
 * If login fails, this function prints an error and never returns.
 * This function does not require super-user privileges.
 */
void
ssh_login(struct ssh *ssh, Sensitive *sensitive, const char *orighost,
    struct sockaddr_storage *hostaddr, u_short port, struct passwd *pw,
    int timeout_ms, const struct ssh_conn_info *cinfo)
{
  char *host;
  char *server_user, *local_user;
  int r;

  local_user = xstrdup(pw->pw_name);
  server_user = options.user ? options.user : local_user;

  /* Convert the user-supplied hostname into all lowercase. */
  host = xstrdup(orighost);
  lowercase(host);

  /* Exchange protocol version identification strings with the server. */
  if ((r = kex_exchange_identification(ssh, timeout_ms,
      options.version_addendum)) != 0)
    sshpkt_fatal(ssh, r, "banner exchange");

  if ((ssh->compat & SSH_BUG_NOREKEY)) {
    logit("Warning: this server does not support rekeying.");
    logit("This session will eventually fail");
  }

  /* Put the connection into non-blocking mode. */
  ssh_packet_set_nonblocking(ssh);

  /* key exchange */
  /* authenticate user */
  debug("Authenticating to %s:%d as '%s'", host, port, server_user);
  ssh_kex2(ssh, host, hostaddr, port, cinfo);
  kex_set_warn_weak_crypto(ssh, options.warn_weak_crypto &&
      !options.kex_algorithms_set);
  if (!options.kex_algorithms_set && ssh->kex != NULL &&
      ssh->kex->name != NULL && options.warn_weak_crypto &&
      !kex_is_pq_from_name(ssh->kex->name)) {
    warn_nonpq_kex();
    ssh->kex->non_pq_kex_warned = 1;
  }
  ssh_userauth2(ssh, local_user, server_user, host, sensitive);
  free(local_user);
  free(host);
}

/* print all known host keys for a given host, but skip keys of given type */
static int
show_other_keys(struct hostkeys *hostkeys, struct sshkey *key)
{
  int type[] = {
    KEY_RSA,
    KEY_ECDSA,
    KEY_ED25519,
    KEY_MLDSA44_ED25519,
    -1
  };
  int i, ret = 0;
  char *fp, *ra;
  const struct hostkey_entry *found;

  for (i = 0; type[i] != -1; i++) {
    if (type[i] == key->type)
      continue;
    if (!lookup_key_in_hostkeys_by_type(hostkeys, type[i],
        -1, &found))
      continue;
    fp = sshkey_fingerprint(found->key,
        options.fingerprint_hash, SSH_FP_DEFAULT);
    ra = sshkey_fingerprint(found->key,
        options.fingerprint_hash, SSH_FP_RANDOMART);
    if (fp == NULL || ra == NULL)
      fatal_f("sshkey_fingerprint fail");
    logit("WARNING: %s key found for host %s\n"
        "in %s:%lu\n"
        "%s key fingerprint %s.",
        sshkey_type(found->key),
        found->host, found->file, found->line,
        sshkey_type(found->key), fp);
    if (options.visual_host_key)
      logit("%s", ra);
    free(ra);
    free(fp);
    ret = 1;
  }
  return ret;
}

static void
warn_changed_key(struct sshkey *host_key)
{
  char *fp;

  fp = sshkey_fingerprint(host_key, options.fingerprint_hash,
      SSH_FP_DEFAULT);
  if (fp == NULL)
    fatal_f("sshkey_fingerprint fail");

  error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
  error("@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @");
  error("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
  error("IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!");
  error("Someone could be eavesdropping on you right now (man-in-the-middle attack)!");
  error("It is also possible that a host key has just been changed.");
  error("The fingerprint for the %s key sent by the remote host is\n%s.",
      sshkey_type(host_key), fp);
  error("Please contact your system administrator.");

  free(fp);
}

/*
 * Execute a local command
 */
int
ssh_local_cmd(const char *args)
{
  char *shell;
  pid_t pid;
  int status;
  void (*osighand)(int);

  if (!options.permit_local_command ||
      args == NULL || !*args)
    return (1);

  if ((shell = getenv("SHELL")) == NULL || *shell == '\0')
    shell = _PATH_BSHELL;

  osighand = ssh_signal(SIGCHLD, SIG_DFL);
  pid = fork();
  if (pid == 0) {
    ssh_signal(SIGPIPE, SIG_DFL);
    debug3("Executing %s -c \"%s\"", shell, args);
    execl(shell, shell, "-c", args, (char *)NULL);
    error("Couldn't execute %s -c \"%s\": %s",
        shell, args, strerror(errno));
    _exit(1);
  } else if (pid == -1)
    fatal("fork failed: %.100s", strerror(errno));
  while (waitpid(pid, &status, 0) == -1)
    if (errno != EINTR)
      fatal("Couldn't wait for child: %s", strerror(errno));
  ssh_signal(SIGCHLD, osighand);

  if (!WIFEXITED(status))
    return (1);

  return (WEXITSTATUS(status));
}

void
maybe_add_key_to_agent(const char *authfile, struct sshkey *private,
    const char *comment, const char *passphrase)
{
  int auth_sock = -1, r;
  const char *skprovider = NULL;

  if (options.add_keys_to_agent == 0)
    return;

  if ((r = ssh_get_authentication_socket(&auth_sock)) != 0) {
    debug3("no authentication agent, not adding key");
    return;
  }

  if (options.add_keys_to_agent == 2 &&
      !ask_permission("Add key %s (%s) to agent?", authfile, comment)) {
    debug3("user denied adding this key");
    close(auth_sock);
    return;
  }
  if (sshkey_is_sk(private))
    skprovider = options.sk_provider;
  if ((r = ssh_add_identity_constrained(auth_sock, private,
      comment == NULL ? authfile : comment,
      options.add_keys_to_agent_lifespan,
      (options.add_keys_to_agent == 3), skprovider, NULL, 0)) == 0)
    debug("identity added to agent: %s", authfile);
  else
    debug("could not add identity to agent: %s (%d)", authfile, r);
  close(auth_sock);
}

Coverage Report

Created: 2026-10-17 06:40